Geometries are optimized at the restricted Hartree Fock (RHF) levelfor closed-shell singlets. Previous work'8 has shown that bond lengthsand angles for TM complexes (involving complexes in a variety ofgeometries and oxidation states and of metals from the entire transitionseries) are predicted to within 1-3% of experiment using the presentcomputational scheme.Vibrational frequencies are calculated at stationary points to identifythem as minima (zero imaginary frequencies) or transition states (oneimaginary frequency). Plotting the imaginary frequencies is used to assesswhich TS connects which reactants and products. In some cases theintrinsic reaction coordinate (IRC) is followed using the steepest descentalgorithm of Ishida et al.,21a with the stabilization method described bySchmidt et al.21bAlthough geometries are accurately predicted at the RHF level,energetics will typically be poor if correlation is ignored. For speciesdescribed well at the RHF level, the correlation contribution can be treatedas a perturbation of the RHF energy and calculated using Moller-Plessetsecond-order perturbation theory (MP2).22 Koga and Morokuma'5 useda similar scheme in their work, as have Krogh-Jespersen et al. in studiesof oxidative addition.23 A simple RHF geometry/MP2 energy schemeyielded good agreement with experimental data for high-valent C-H-activating systems, both in terms of absolute numbers and trends.1sa-cResults and Discussion1. Reactants. The geometry of methane is well-known.24Reactant complexes are three-coordinate Ir(X)(PH3)2 complexes(X = H, Cl), models of the putative 14-electron, C-H-activatingspecies in dehydrogenation catalysts.81,12a,b As in previouscomputational work'13,5- 7 we have focused on the lowest energy,singlet surface. Geometries for bis(phosphino)Ir' chloride andbis(phosphino)IrI hydride minima (C2 symmetry) are shown in1. As expected for a d8 ML3 complex, the geometry is T-shaped.25